The invention relates to vacuum bearing structures and methods of supporting movable members, particularly in apertures provided in walls of vacuum chambers. In a preferred embodiment, the invention is applied in the field of ion implanters.
There is often a need to mount movable members within a vacuum chamber, in particular when the movement has to be communicated from outside the vacuum chamber. Electrically powered actuators, such as electric motors, do not work well in a vacuum and so these are normally installed external to a vacuum chamber and some vacuum seal arrangement provided which will permit the required motion of the member within the vacuum chamber to be communicated from outside. Similarly, hydraulic or other fluid pressure actuation is normally to be avoided within a vacuum chamber.
Rotary vacuum seals are well known, for example ferro-magnetic fluid seals. Seals accommodating a linear motion are usually more problematic and prior art solutions include flexible bellows-type seals.
Vacuum seals accommodating a linear motion have been proposed using a combination of an air bearing and a differentially-pumped vacuum seal located between the air bearing and the interior of the vacuum chamber. An earlier proposal for such an arrangement is contained in WO 82/02235 (Fox). An arrangement of this kind for use in an ion implanter is disclosed in U.S. Pat. No. 5,898,179 (Smick et al). This U.S. patent also describes use of the air bearing seal combination in a relatively large radius rotary arrangement.
There is obviously a conflict between the requirements of a gas or air bearing and the need to prevent gas leakage into the vacuum chamber.
The arrangement disclosed in the above U.S. patent to Smick et al can work well, but there is still a requirement to reduce the loading on the differential pumping and to ensure minimal gas leakage into the vacuum chamber through the differentially-pumped seal.
Accordingly, it is an object of the present invention to improve the performance of the above-referred vacuum bearing structures using a combination of a gas bearing and a differentially-pumped vacuum seal for motion feedthrough into vacuum chambers.
In one aspect the present invention provides a vacuum bearing structure comprising: a vacuum chamber including a wall having an aperture, a movable member arranged to extend through said aperture, one of said vacuum chamber wall and said movable member having a first bearing surface provided thereon, the other of said vacuum chamber wall and said movable member having a second bearing surface provided thereon, said first and second bearing surfaces extending transverse to the line of action of external pressure acting on said movable member to urge said surfaces towards each other, said first and second bearing surfaces permitting a predetermined movement of said movable member relative to said chamber wall, said first bearing surface being provided by porous gas-permeable material facing said second bearing surface, said porous material having a first region which is porous to gas flow through the thickness of the material and a second, inner region between said first region and the interior of the vacuum chamber, at least one gas supply plenum beneath said first region for a supply of bearing gas under pressure to percolate through said porous first region to apply a bearing pressure to said second bearing surface opposing the action of said external pressure, an exhaust groove through said porous material and positioned between said first region and said second, inner region thereof, and an exhaust plenum connecting to said exhaust groove for allowing bearing gas to escape from between said bearing surfaces.
The use of porous material for gas bearing arrangements is itself not new. An article entitled xe2x80x9cAir bearings take offxe2x80x9d published in FT Design, April 1999, describes the use of porous graphite material for air bearings in a number of applications. However, an important feature of this aspect of the present invention is that the porous material covering one of the bearing surfaces is divided into a first region and an inner second region. Bearing gas is supplied to the first region from a plenum beneath the first region to provide the supporting gas for the bearing and an exhaust groove is provided through the porous material between the first region and the inner second region so that bearing gas escaping from the bearing surfaces towards the interior can be exhausted away to atmosphere. The inner second region of porous material can then be used to provide the vacuum seal arrangement.
This design is particularly though not exclusively applicable to relatively large scale vacuum bearings, such as may be used for example in the above-referred U.S. patent to Smick et al. In such arrangements, it is very important that the bearing surfaces themselves and also the inner vacuum seal surfaces are machined accurately flat, and usually coplanar. By making both the first region of the surface, which provides the gas bearing support, and the inner second region of the surface, which provides the vacuum seal region, of the same porous material, these can be machined simultaneously to a very high degree of mutual flatness.
At least one continuous differential pumping groove is preferably provided through said second region of the porous material, between said exhaust groove and the interior of the vacuum chamber. Lands of the first bearing surface are thus provided in the second region of the porous material on each side of the differential pumping groove. A differential pumping plenum is also provided connecting to the differential pumping groove for connection to a vacuum pump.
At least the first region of the porous material may have a surface facing said second bearing surface which is impregnated to provide a substantially uniform porosity at said surface which is lower than the porosity of the porous material. By xe2x80x9clower porosityxe2x80x9d, it is meant that the resistance to gas flow is higher.
In another aspect the invention provides a vacuum bearing structure comprising a vacuum chamber including a wall having an aperture, a movable member arranged to extend through said aperture, a bearing between said vacuum chamber wall and said movable member allowing a predetermined movement therebetween, first and second spaced opposed sealing surfaces on said vacuum chamber wall and said movable member, a continuous differential pumping groove in said first sealing surface, respective lands of said first sealing surface on each side of said differential pumping groove, a differential pumping plenum extending beneath said differential pumping groove and having a width greater than the width of said differential pumping groove so that at least one edge of the differential pumping groove is cantilevered over the underlying differential pumping plenum, and bridging elements crossing over said differential pumping element under said differential pumping groove to support said at least one cantilevered edge of the differential pumping groove.
This construction is especially useful when the first bearing surface containing the differential pumping groove is formed of a layer of porous material overlying a solid substrate. It is generally important for differential pumping arrangements for the lands on either side of a pumping groove to be as extensive as possible in the direction towards the interior of the vacuum chamber. Thus, the differential pumping grooves themselves should be relatively narrow. However, to ensure good gas conductivity from the differential pumping groove to the vacuum pump, the plenum connected to the differential pumping groove should itself be as large as possible. This construction results in at least one edge of the differential pumping groove extending in a cantilevered fashion over the underlying plenum.
When the structure is formed, the first bearing surface must be machined flat to a very high tolerance. The grinding or machining action on the surface would tend to cause a small deviation of the cantilevered edge or edges of the differential pumping groove, if these are unsupported. Once the load on the first bearing surface from the grinding or machining action is removed, the cantilevered edges recover, with the result that there is a slight excess of material at the edges, above the plane of the accurately flat surface of the first bearing surface.
By providing the bridging structure described above, the edges of the bearing surface are supported during the grinding action to very much reduce this effect.
The invention also provides a method of supporting a movable member in an aperture provided in a wall of a vacuum chamber, wherein one of said vacuum chamber wall and said movable member has a first bearing surface and the other has a second bearing surface, and the first and second surfaces extend transverse to the line of action of external pressure acting on said movable member to urge said first and second surfaces towards each other and permit a predetermined movement of said movable member relative to said chamber wall, the method comprising the steps of providing a porous gas-permeable material as said first surface facing said second surface, said porous material having a first outer region which is porous to gas flow through the thickness of the material and a second, inner region, applying gas under pressure via a gas supply plenum to said first region to permit gas under pressure to percolate through said first region to apply bearing pressure to said second surface opposing the action of said external pressure, and exhausting gas to atmosphere via an exhaust channel between said outer and inner regions.
Examples of the invention will now be described with reference to the drawings.